## Digital Communications Questions and Answers Part-3

1. Power spectral density function is a?
a) Real and even function
b) Non negative function
c) Periodic
d) All of the mentioned

Explanation: A power signal is periodic signal and its function is a real even and non negative function as per the definition.

2. Energy spectral density defines
a) Signal energy per unit area
b) Signal energy per unit bandwidth
c) Signal power per unit area
d) Signal power per unit bandwidth

Explanation: Energy spectral density gives the signal energy equal to the area under the waveform energy spectral density versus frequency curve.

3. Power spectrum describes distribution of _________ under frequency domain.
a) Mean
b) Gaussian
c) Variance
d) None of the mentioned

Explanation: Power spectrum gives the distribution of variance of a signal in the frequency domain, sampled into spectral components.

4. How can power spectral density of non periodic signal be calculated?
a) By integrating
b) By truncating
c) By converting to periodic
d) None of the mentioned

Explanation: A power signal usually is a periodic signal. But power spectral density of non periodic signal can be calculated by truncating it and observing it in the range of (-T/2,T/2).

5. According to Parseval’s theorem the energy spectral density curve is equal to?
a) Area under magnitude of the signal
b) Area under square of the magnitude of the signal
c) Area under square root of magnitude of the signal
d) None of the mentioned

Explanation: According to Parseval's theorem the energy spectral density function can be given as equal to the square of the magnitude of the signal x(t).

6. What is Wiener-Khinchin theorem?
a) Spectral density and auto-covariance makes a fourier transform pair
b) Spectral density and auto-correlation makes a fourier tranform pair
c) Spectral density and variance makes a fourier tranform pair
d) None of the mentioned

Explanation: According to the theorem spectral density of a signal x(t) and auto correlation function makes a fourier transform pair.

7. Spectogram is the graph plotted against?
a) Time domain
b) Frequency domain
c) Frequency & Time domain
d) None of the mentioned

Explanation: Spectral density function is plotted against frequency domain and spectogram is a graph which is plotted against time domain.

8. Autocorrelation is a function which matches
a) Two same signals
b) Two different signal
c) One signal with its delayed version
d) None of the mentioned

Explanation: Autocorrelation is a function that matches a signal with its delayed version.

9. Autocorrelation is a function of
a) Time
b) Frequency
c) Time difference
d) Frequency difference